Various light emitting materials including gallium nitride and solid solution thereof that show an emission wavelength of about 300 nm have been proposed in recent years in the course of development of high luminance UV light emitting materials for practical applications. There is a large demand for solid state light emitting materials showing a short emission wavelength including those that can be used for writing data highly densely on recording mediums and those that can be used to take the place of mercury-arc lamps containing mercury that is harmful to human bodies.
The requirements that need to be met by materials to be used for high luminance light emitting devices in the far-UV region (which is defined as wavelength range between 200 and 240 nm herein) include that such materials show a wide band gap and that they are chemically stable and desirably semiconductors of the direct transition type. The group of researchers including the inventors of the present invention has paid intensive efforts of developing materials that emit far-UV light to find as a result that highly purified crystalline boron nitride can be obtained by treating boron nitride in a solvent under high-pressure and the obtained high-purity hexagonal boron nitride monocrystal (the monocrystal is referred to as hBN hereinafter) has a band gap at or near 5.8 eV and a strong emission peak in a far-UV region at or near the wavelength of 200 nm. A patent application has been filed for the above research achievement, which is also published in a research paper (see Patent Documents 1 through 3 and Non-Patent Document 1).
However, the bond of nitrogen atoms and boron atoms is based on sp2 bonds in the obtained hBN and the hBN has a disadvantage that the designed profile of the hBN can be easily broken when an impetus is given mechanically and/or physically and is accompanied by a problem that the emission wavelength region varies before and after the breakage. Such problems are far from desirable as a matter of course when designing a light emitting device that is stable and shows a high emission efficiency. In other words, a far-UV luminescence material that can be handled with ease and morphologically stable is desired. Such morphological instability is experienced also in material design utilizing the proper high temperature stability of boron nitride. In short, it is desirable to put a target material in a state that is not subjected to morphological variations and convenient for use.
Sp3-bonded boron nitride described in Patent Document 4 is a desirable material from the above-described point of view. However, the described method of manufacturing the material is accompanied by drawbacks including that it is complex from the viewpoint of reproducibility and indispensably requires the use of an ultraviolet laser or a pulse laser, which is highly costly. Additionally, it is very difficult to manufacture boron nitride that is homogeneous in terms of light luminescence characteristics by means of the described method, which is a vapor phase growth method.